MadSci Network: Genetics

Re: Why would an Asian have blue eyes?

Date: Tue Jan 29 18:20:47 2002
Posted By: Gabriel Vargas M.D.,Ph.D., Post-doc/Fellow, Neurosciences/Psychiatry
Area of science: Genetics
ID: 1009528130.Ge

Most  models of eye inheritance postulate that there are 3 genes involved
in eye color determination and that brown eyes are dominant while blue eyes
are recessive so that if one has an allele (a version of a gene ) that
determines brown eyes that person
will have brown eyes.  To have blue eyes you must have two blue eye alleles.
 It is possible for 2 brown eyed people (both carrying the recessive blue
eye allele) to have blue-eyed children.  In fact it is not rare.  It is
also possible for blue eyed alleles to be carried for several generations
without having blue eyed people until 2 people with blue eyed alleles
marry. It is not thought that eye color affects vision unless
one has no eye pigmentation such as albinos in which case they do have
visual problems. I have included additional information about eye genetics
from a website  affiliated with Oregon public schools:

hope this helps,
gabriel vargas md/phd

Eye color is a physical trait that is genetically determined. A favorite
pastime for many expectant parents is documenting the eye color of
grandparents and extended family members to attempt to predict the eye
color of their baby. Any discussion of the inheritance of eye color,
requires a review of genetics. In basic terms, eye color is determined by
the amount of a pigment called melanin that is in the iris of the eye.
Brown eyes have lots of pigment, blue eyes very little. The amount of
pigment is determined by a number of genes controlling pigment production.
Generally speaking, brown is dominant, meaning that if one parent has brown
eyes and the other has blue eyes, the baby will most likely have brown
eyes. But the situation is really much more complicated than that. For a
more complete description, read on. What is a gene and what does it have to
do with eye color?Humans have a diploid number of 46 chromosomes (23 pair)
located in the cell nucleus. In the process called meiosis, sperm and egg
cells are produced that have the haploid number (23) of chromosomes. During
fertilization of the egg by the sperm the diploid number of 46 is restored.
During the course of pregnancy, the fertilized egg (zygote) undergoes a
complex series of changes including multiple cell divisions and
differentiation of cells into the different organ systems. The result is a
baby whose cells each have 46 chromosomes. One chromosome of each pair is
inherited from the baby's father and the other from the mother. A gene is a
length of DNA on a chromosome that does something particular for an
organism. It is the basic unit of heredity, determining the observable
characteristics, or phenotype, of an individual. The actual genetic makeup
of an individual is referred to as its genotype. Genes are arranged in a
linear fashion on the chromosome, much like beads on a string. The
arrangement of genes along a chromosome is the same for both members of a
chromosome pair. Therefore genes also occur in pairs. If genes occur in
pairs are both genes alike?Genes may occur in different forms called
alleles so the appearance of any characteristic is dependent on the types
of alleles present. A person who carries two of the same alleles is called
homozygous for that gene. If two different alleles are present the person
is heterozygous for that gene. In a heterozygous gene pair, one allele may
be expressed over the other. The allele that is expressed is called the
dominant allele. The allele that is not expressed is called the recessive
allele. Dominance and recessiveness refer to which allele is expressed when
the two occur together. There is nothing abnormal or defective implied in
the terms. Examples of dominant/recessive inheritance are many of the
famous genetic experiments conducted by Gregor Mendel. Mendel bred pea
plants and found that crosses between homozygous purple flower plants and
homozygous white flower plants always produced plants with purple flowers.
In the process of incomplete dominance, a heterozygous gene pair produces a
characteristic that is intermediate between the characteristic produced by
the homozygous condition of each allele. An example of this occurs when a
red snapdragon is crossed with a white one resulting in a plant with pink
flowers. In a third process called codominance, the affect of each allele
is expressed simultaneously. An example of codominance is the human AB
blood group system in which a person with an A allele and a B allele will
have type AB blood. I have heard that a single pair of genes determines
what color my eyes are. Is this true?At one time scientists thought that a
single gene pair, in a dominant/recessive inheritance pattern, controlled
human eye color. The allele for brown eyes was considered dominant over the
allele for blue eyes. The genetic basis for eye color is actually far more
complex. At the present, three gene pairs controlling human eye color are
known. Two of the gene pairs occur on chromosome pair 15 and one occurs on
chromosome pair 19. The bey 2 gene, on chromosome 15, has a brown and a
blue allele. A second gene, located on chromosome 19 (the gey gene) has a
blue and a green allele. A third gene, bey 1, located on chromosome 15, is
a central brown eye color gene. Geneticists have designed a model using the
bey 2 and gey gene pairs that explains the inheritance of blue, green and
brown eyes. In this model the bey 2 gene has a brown and a blue allele. The
brown allele is always dominant over the blue allele so even if a person is
heterozygous (one brown and one blue allele) for the bey 2 gene on
chromosome 15 the brown allele will be expressed. The gey gene also has two
alleles, one green and one blue. The green allele is dominant to the blue
allele on either chromosome but is recessive to the brown allele on
chromosome 15. This means that there is a dominance order among the two
gene pairs. If a person has a brown allele on chromosome 15 and all other
alleles are blue or green the person will have brown eyes. If there is a
green allele on chromosome 19 and the rest of the alleles are blue, eye
color will be green. Blue eyes will occur only if all four alleles are for
blue eyes. This model explains the inheritance of blue, brown and green
eyes but cannot account for gray, hazel or multiple shades of brown, blue,
green and gray eyes. It cannot explain how two blue-eyed parents can
produce a brown-eyed child or how eye color can change over time. This
suggests that there are other genes, yet to be discovered, that determine
eye color or that modify the expression of the known eye color genes. How
does a gene, a section of a chromosome in the cell nucleus, make my eyes a
particular color?The exact color of the human eye is determined by the
amount of a single pigment called melanin that is present in the iris of
the eye. Melanin is a dark brown pigment that is deposited on the front
surface of the iris. If a lot of melanin is present, the eye will appear
brown or even black. If very little melanin is present the iris appears
blue. Intermediate amounts of melanin produces gray, green, hazel or
varying shades of brown. Genes work by directing the production of enzymes,
chemicals that control all of the processes that occur in our body. Eye
color genes, through the enzymes they produce, direct the amount and
placement of melanin in the iris. Newborn babies all have blue eyes because
at the time of birth they haven't begun to produce melanin in their irises.
A baby's eyes may change to green, brown or other colors as melanin
production begins. Albinos have no pigment in their irises so the blood
vessels in the back of the eye reflect light making the eyes look pink.
Albinos also lack melanin in their skin and hair. Since albinism is caused
by a recessive allele, two normal parents may produce an albino. An albino
can have normal offspring if the other parent is normal for melanin
production. Where can I find out more about human genetics?

The following WEB sites provide more information about human eye color
genetics and other human genetics topics.
 Eye Color by OMIM, Online Mendelian Inheritance in Man:

 Human Eye Color Genes by Athro Ltd.:

Eye Color Genetics by the Franklin Institute Online:

Anatomy by the Franklin Institute Online:
Human Genetics
in the On-Line  

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